Many clinically common chronic neurodegenerative diseases, such as Parkinson's disease, Alzheimer's disease, Huntington's disease, and amyotrophic lateral sclerosis, are resulted from the degeneration and death of neurons in the central nervous system (CNS). Neurons, unlike epithelial and liver cells, cannot regenerate quickly. Although there are indeed stem cells present in the CNS, which can replicate and differentiate either under normal conditions or when neurons are damaged, their replication is quite slow and the proportion of neuronal differentiation is very low. Most of the stem cells in the CNS differentiate into glial cells and thus cannot rescue the neurodegenerative diseases.
Neuron transplantation has provided a direction for treating neurodegenerative diseases. Researchers have been trying to generate neurons from various stem cells both in vitro and in vivo. Stem cells are believed to possess certain characteristics, including self-renewal, pluripotency, proliferation, longevity and differentiation. Fetuses, umbilical cord blood and bone marrow are the most common sources of human stem cells for clinical applications. In fetuses, stem cells applicable in treating pathological changes of the central nervous system were mainly embryonic stem cells of blastocyst and neural stem cells (R. L. Rietze et al., 2001, Nature 412: 736-739). Researches have shown that after transplantation of embryonic stem cells (J. W. McDonald et al., 1999, Nature Medicine 5: 1410-1412) or neural stem cells (Q. L. Cao et al., 2001, Experimental Neurology 167: 48-58) for repairing a spinal trauma, these cells mainly differentiated into astrocytes and oligodendrocytes and only extremely few cells turned into neuronal cells. Apart from the difficulty in retrieving, the low survival rate after transplantation, and the extremely low rate for turning into neural cells, transplantation of fetus tissues is also hindered by disputes in the aspects of religion, law and ethic and therefore is not available to all.
The repair of damaged neural cells by direct implantation of haematopoietic stem cells of umbilical cord blood has yet to be studied. However, in vitro model showed that haematopoietic stem cells of umbilical cord blood cultured in 0.001% of β-mercaptoethanol (BME) could be transformed into precursors of neural cells, and then 10% of these cells could be further induced to differentiate into neural cells and glial cells (J. R. Sanchez-Ramos et al., 2001, Experimental Neurology 171: 109-115; and Y. Ha et al., 2001, Neuroreport 12(16): 3523-3527). Since the conversion rate of umbilical cord blood stem cells into neuronal cells was relatively low, the research and application of umbilical cord blood mainly focused on haematological diseases.
Bone marrow contains hematopoietic stem cells that provide a continuous source for progenitors of red cells, platelets, monocytes, granulocytes, and lymphocytes. Moreover, bone marrow also contains cells that meet the criteria for stem cells of nonhematopoietic tissues. Hematopoietic stem cells from adult bone marrow have been reported to be able to differentiate into both microglia and macroglia in the brains of mice (M. A. Eglitis et al., 1997, Proc. Natl. Acad. Sci. 94: 4080-4085). Nonhematopoietic stem cells from bone marrow stroma have been referred to as bone marrow stromal cells (BMSCs) or bone marrow mesenchymal cells. BMSCs are capable of differentiating into osteogenic, chondrogenic, adipogenic, myogenic and other lineages in vitro (M. F. Pittenger et al., 1999, Science 284: 143-147 and B. E. Petersen et al., 1999, Science 284: 1168-1170). Recently, BMSCs (S. A. Azizi et al., 1998, Proc. Natl. Acad. Sci. 95: 3908-3913; and G. C. Kopen et al., 1999, Proc. Natl. Acad. Sci. 96: 10711-10716) have been reported to be able to differentiate into neurons in vivo.
All the above methods of generating neurons from multipotent cells have one common problem: most of the multipotent stem cells differentiate into glial cells (astrocytes, oligodendrocytes etc.) instead of neurons. Only for in vitro culture of BMSCs, it has been observed that BME has a significant effect on neuronal differentiation, which induces about 80% of the cultured BMSCs to differentiate into neurons (D. Woodbury et al., 2000, Journal of Neuroscience Research 61: 364-370). However, BME is toxic for proteins and thus unsuitable for use in human bodies (K. White et al., 1973, Journal of Pharmaceutical Sciences 62: 237-241). Therefore, there is still a need for an efficient method to transform stem cells into neurons.